Electrochemical cells provide electrical energy that powers a host of electronic devices that range from medical devices to electronic devices utilized in gas and oil exploration. Among these many devices powered by electrochemical cells are electronic devices such as pipeline inspection gauges that are used in down-hole petroleum exploration. Such devices generally require the delivery of a significant amount of current over long periods of time in relatively harsh environments. Thus, these devices typically require the use of electrochemical cells that comprise an increased delivery capacity and an increased rate of charge delivery. In addition, these cells must be able to safely operate in harsh environments that may comprise elevated temperatures, increased atmospheric pressures, caustic environments, explosive atmospheres, or combinations thereof.
As defined herein, “delivery capacity” is the maximum amount of electrical current that can be drawn from a cell under a specific set of conditions. The terms, “rate of charge delivery” and “rate capability” are defined herein as the maximum continuous or pulsed output current a battery can provide per unit of time. Thus, an increased rate of charge delivery occurs when a cell discharges an increased amount of current per unit of time in comparison to a similarly built cell, but of a different anode and/or cathode chemistry.
Cathode chemistries such as carbon monofluoride (CFx) have a relatively high energy density compared to liquid cathode lithium-oxyhalide systems such as lithium-thionyl chloride, lithium-sulfuryl chloride and others. CFx cathode material is generally known to have a discharge capacity of about 875 mAh/g, which is well suited for powering electrical devices over long periods of time. However, electrochemical cells constructed with cathodes comprised of carbon monofluoride are generally considered to exhibit a relatively “low” rate capability. For example, electrochemical cells constructed with lithium anodes and CFx cathodes typically exhibit rate capabilities from about 0.5 mA/cm2 to about 3 mA/cm2. As such, electrochemical cells constructed with Li/CFx couples are generally well suited for powering electrical devices over long periods of time at a relatively low rate capability.
In contrast, electrochemical cells constructed with lithium anodes and cathodes comprising a liquid lithium-oxyhalide, such as lithium-thionyl chloride and lithium-sulfuryl chloride, are generally considered to exhibit a relatively “high” rate capability. Lithium cells constructed with liquid lithium-oxyhalide cathodes, in contrast to CFx cathodes, generally exhibit rate capabilities that range from about 0.5 mA/cm2 to about 10 mA/cm2. As such, lithium electrochemical cells constructed with cathodes comprised of liquid lithium-oxyhalide are generally well suited to power electronic devices used in down-hole oil and gas extraction that require an increased rate capability.
However, the liquid lithium-oxyhalide cathode material contained within these cells is generally considered to be caustic and may become volatile, principally if exposed to water. If such a cell comprising liquid lithium-oxyhalide were to rupture, the cathode material might cause a safety issue particularly within a petroleum rich environment. Therefore, what is desired is a cathode material and electrochemical cell thereof that comprises a “high” discharge capacity in addition to an increased rate capability that can safely operate in harsh environments. Such an electrochemical cell would be well suited for powering additional electronic devices that require an increased charge capacity with an increased discharge rate in harsh environments such as those found within petroleum wells.
The applicants, therefore, have developed a new CFx cathode material formulation and cathode thereof that provides a lithium electrochemical cell with increased rate capability. Specifically, the applicants have developed a cathode formulation in which a first cathode active material comprising a metallic phosphate is mixed with a second cathode active material comprising a carbonaceous material to thereby increase the cell's rate capability. Specifically, the addition of the metallic phosphate within the carbonaceous material structure is designed to increase the rate capability of the lithium cell. Thus, a cathode composed of a mixture of CFx and a metallic phosphate additive of the present invention when constructed within an electrochemical cell having a lithium anode is well suited for powering a variety of electrical devices that require a “high” discharge capacity with an increased rate capability. In addition, CFx is a solid and chemically stable cathode material that is generally considered to be less reactive than liquid-oxyhalide chemistries. Therefore, the possibility that the cathode may chemically react uncontrollably if exposed to environments outside the cell casing is reduced.